Eddy current system for in-situ profile measurement

ABSTRACT

An eddy current monitoring system may include an elongated core. One or more coils may be coupled with the elongated core for producing an oscillating magnetic field that may couple with one or more conductive regions on a wafer. The core may be translated relative to the wafer to provide improved resolution while maintaining sufficient signal strength. An eddy current monitoring system may include a DC-coupled marginal oscillator for producing an oscillating magnetic field at a resonant frequency, where the resonant frequency may change as a result of changes to one or more conductive regions. Eddy current monitoring systems may be used to enable real-time profile control.

TECHNICAL FIELD

This disclosure relates to semiconductor processing, and moreparticularly to systems and techniques for monitoring one or moreconductive regions during semiconductor processing.

BACKGROUND

An integrated circuit is typically formed on a substrate (e.g. asemiconductor wafer) by the sequential deposition of conductive,semiconductive or insulative layers on a silicon wafer, and by thesubsequent processing of the layers.

One fabrication step involves depositing a filler layer over anon-planar surface, and planarizing the filler layer until thenon-planar surface is exposed. For example, a conductive filler layercan be deposited on a patterned insulative layer to fill the trenches orholes in the insulative layer. The filler layer is then polished untilthe raised pattern of the insulative layer is exposed. Afterplanarization, the portions of the conductive layer remaining betweenthe raised pattern of the insulative layer form vias, plugs and linesthat provide conductive paths between thin film circuits on thesubstrate. In addition, planarization may be used to planarize thesubstrate surface for lithography.

Chemical mechanical polishing (CMP) is one accepted method ofplanarization. This planarization method typically requires that thesubstrate be mounted on a carrier or polishing head. The exposed surfaceof the substrate is placed against a rotating polishing disk pad or beltpad. The polishing pad can be either a “standard” pad or afixed-abrasive pad. A standard pad has a durable roughened surface,whereas a fixed-abrasive pad has abrasive particles held in acontainment media. The carrier head provides a controllable load on thesubstrate to push it against the polishing pad. A polishing slurry,including at least one chemically-reactive agent, and abrasive particlesif a standard pad is used, is supplied to the surface of the polishingpad.

During semiconductor processing, it may be important to determine one ormore characteristics of the substrate or layers on the substrate. Forexample, it may be important to know the thickness of a conductive layerduring a CMP process, so that the process may be terminated at thecorrect time. A number of methods may be used to determine substratecharacteristics. For example, optical or capacitance sensors may be usedfor in-situ monitoring of a substrate during chemical mechanicalpolishing. Alternately (or in addition), an eddy current sensing systemmay be used to induce eddy currents in a conductive region on thesubstrate to determine parameters such as the local thickness of theconductive region.

SUMMARY

The current disclosure provides systems and techniques for obtaininghigh spatial resolution eddy current measurements, and for obtainingeddy current measurements with a high signal to noise ratio. In general,in one aspect an eddy current sensing system includes an elongated core.The elongated core has a length greater than a width. A coil woundaround a protrusion of the elongated core produces a time-dependentmagnetic field to induce eddy currents in a conductive region such as afirst region of a conductive layer on a wafer. The first region in whichthe eddy currents are induced is elongated as well, having a lengthgreater than a width.

The elongated core may be positioned proximate to a wafer carrier of asemiconductor processing apparatus. For example, the core may bepositioned at least partially in a platen of a chemical mechanicalpolishing apparatus, so that a top surface of the protrusion is to bepositioned proximate to a top surface of a polishing pad coupled withthe platen. The elongated core may comprise a ferrite material such as aMnZn ferrite, NiZn ferrite, or other ferrite. The elongated core may becoated with a material such as parylene.

In general, in one aspect, a method comprises processing a conductivelayer on a wafer using a plurality of processing parameters. Forexample, a metal layer may be polished using a CMP apparatus, and theprocessing parameters may include a slurry composition, as well as apressure profile applied by a polishing head.

Eddy currents may be induced in a first region of a conductive layer ona wafer, where the first region has a length greater than a width. Theeddy currents may be induced using a time-dependent magnetic fieldgenerated by a current in a coil wound around an elongate core.

Thickness data may be acquired for the conductive layer in the firstregion based on the eddy currents induced in the first region. Eddycurrents may be induced in a second region of the conductive layer, andmeasured thickness data for the second region may be acquired. Themeasured thickness data for the first and second regions can be comparedto a desired thickness profile to determine a profile error. If theprofile error exceeds a minimum desired error, one or more processingparameters may be changed.

The width of the first region may be about a millimeter or less, so thatthe spatial resolution of the system at the first region is on the orderof about a millimeter. The width may be between about one millimetersand about three millimeters.

In general, in one aspect, a chemical mechanical polishing apparatuscomprises a direct current (DC) coupled marginal oscillator to generatea time-dependent current in a coil, the coil to generate atime-dependent magnetic field to couple with a portion of a conductiveregion on a wafer. The marginal oscillator may comprise a firsttransistor and a second transistor forming a long-tailed pair. Themarginal oscillator may comprise a third transistor coupled with thefirst transistor to provide DC feedback through a base of the firsttransistor.

The marginal oscillator may generate a time-dependent drive current atthe resonant frequency of a circuit comprising the coil coupled with acore and a capacitor. The third transistor may provide the directcurrent feedback to the base of the first transistor to cause themarginal oscillator to generate the time-dependent drive current suchthat a potential difference across the coil and the capacitor ismaintained at a generally constant amplitude.

In general, in one aspect, a method may include generating atime-dependent drive current at a resonant frequency of a circuitcomprising a coil coupled with a core and a capacitor, thetime-dependent current generated by a marginal oscillator having a firsttransistor and a second transistor comprising a long-tailed pair. Themethod may include inducing eddy currents in a first region of aconductive layer on a wafer, wherein the eddy currents are induced by atime-dependent magnetic field produced by the coil. The method mayinclude determining an amplitude of a potential difference across thecoil and the capacitor and adjusting the time dependent drive currentbased on direct current feedback from a third transistor coupled to thebase of the first transistor to maintain a desired amplitude of thepotential difference. The method may include determining one or moreparameters of the first region based on the time-dependent drivecurrent.

The details of one or more implementations are set forth in theaccompanying drawings and the description below. Other features andadvantages of the invention will be apparent from the description anddrawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 shows a method to implement real-time profile control.

FIGS. 2A and 2B show schematic diagrams of embodiments of an eddycurrent monitoring system.

FIGS. 3A and 3B show an embodiment of an eddy current monitoring systemfor improved linearity and signal to noise ratio.

FIG. 4 shows a method that may be used to determine a polishing endpointor conductive layer thickness, according to an embodiment.

FIG. 5 is a top view of an embodiment of a chemical mechanical polishingapparatus including an eddy current monitoring system.

FIGS. 6A and 6B show side and top views of an embodiment of an elongatedcore for use in an eddy current monitoring system.

FIGS. 7A to 7C show side and top views of another embodiment of anelongated core for use in an eddy current monitoring system.

FIGS. 8A through 8C show embodiments of core shielding that may be used.

FIGS. 9A and 9B show top and side views of a chemical mechanicalpolishing apparatus using an elongated core, according to an embodiment.

FIGS. 10A and 10B show top views of a chemical mechanical polishingapparatus using an elongated core, according to an embodiment.

FIG. 11 shows a side view of a core positioned proximate to a polishingpad, according to an embodiment.

FIG. 12 is a schematic exploded perspective view of an embodiment of achemical mechanical polishing apparatus.

FIG. 13 is a side view illustrating the positioning of a core withrespect to a platen, according to an embodiment.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

In some semiconductor processes, it may be important to know thethickness of a conductive region on the substrate. For example, in orderto determine an endpoint of a metal chemical mechanical polishingprocess, the thickness of the metal layer may need to be monitored. Thepolishing process may be terminated based on measurements related to thethickness of the metal layer.

The thickness of a conductive material may be measured at differentregions on the substrate such as a wafer. For example, the thickness ofa metal layer at different regions on a wafer may be monitored to ensurethat processing is proceeding uniformly across the wafer. Thicknessinformation for regions of the wafer (which collectively may be referredto as a “profile” of the wafer) may then be used to adjust processingparameters in real time to obtain desired cross-wafer uniformity. Forexample, in a chemical mechanical polishing process, the thickness of ametal layer at different regions on the wafer may be monitored, anddetected non-uniformities may cause the CMP system to adjust polishingparameters in real time. Such profile control may be referred to as realtime profile control (RTPC).

FIG. 1 shows a method 100 that may be used to implement RTPC duringsemiconductor processing. A conductive layer on the wafer may beprocessed (110). For example, a copper layer on a wafer may be polishedwith a CMP apparatus including a multi-zone head. While the wafer isbeing polished, profile data may be obtained for a region on the wafer(120). For example, eddy current data related to the thickness of aportion of the copper layer coupled with a magnetic field produced by aneddy current sensing system may be obtained during polishing.

The profile data may be processed (130). For example, signal processingalgorithms may be used to equate eddy current measurements withparticular regions of the wafer. The processed profile data may then becompared to desired profile data to determine if a profile error isgreater than a minimum acceptable error (150). If it is not, theprocessing parameters may be unchanged, and further profile data may beobtained for a different region on the wafer (160). For example, an eddycurrent sensor may be translated with respect to the wafer, so thatprofile information is obtained for regions at different radialdistances from the center of the wafer. Note that the process ofobtaining and processing data, shown as separate discrete steps fordifferent regions of the wafer in FIG. 1, may occur generallycontinuously and concurrently, with data acquisition occurring ontimescales that are short compared to relative translation of an eddycurrent sensor with respect to a wafer.

If the error is greater than a minimum acceptable error, one or moreprocess variables may be changed (170). For example, the CMP system maymake an incremental change to a variable such as the pressure of one ormore of the zones in the multi-zone head, in order to improve polishinguniformity (thus subsequently reducing the measured profile error).

As noted above, profile information may be obtained using eddy currentsensing. With eddy current sensing, an oscillating magnetic fieldinduces eddy currents in a conductive region on the wafer. The eddycurrents are induced in a region that is coupled with magnetic fluxlines generated by the eddy current sensing system. FIG. 2A shows aschematic of a portion of an eddy current sensing system 200. System 200includes a drive coil 210 for generating an oscillating magnetic field220, which may couple with a conductive region 230 of interest (e.g., aportion of a metal layer on a semiconductor wafer). Drive coil 210 iswound around a core 205, which may be formed of a ferrite material suchas a MnZn or NiZn ferrite. Core 205 may be a generally cylindricallysymmetric core, or may be an elongated core such as that shown in FIGS.6A and 6B or FIGS. 7A and 7B, and as described below.

Oscillating magnetic field 220 generates eddy currents locally inconductive region 230. The eddy currents cause conductive region 230 toact as an impedance source in parallel with a sense coil 240 and acapacitor 250. As the thickness of conductive region 230 changes, theimpedance changes, resulting in a change in the Q-factor of the system.By detecting the change in the Q-factor, the eddy current sensingmechanism can sense the change in the strength of the eddy currents, andthus the change in thickness of the conductive region. Therefore, eddycurrent sensing systems may be used to determine parameters of theconductive region, such as a thickness of the conductive region, or maybe used to determine related parameters, such as a polishing endpoint.Note that although the thickness of a particular conductive region isdiscussed above, the relative position of core 205 and the conductivelayer may change, so that thickness information for a number ofdifferent conductive regions is obtained.

In some implementations, a change in Q-factor may be determined bymeasuring an eddy current amplitude as a function of time, for a fixeddrive frequency and amplitude. An eddy current signal may be rectifiedusing a rectifier 260, and the amplitude monitored via an output 270.Alternately, a change in Q-factor may be determined by measuring an eddycurrent phase as a function of time. FIG. 2B shows a system 280 formonitoring the phase as a function of time using a phase detector 290.

System 200 of FIGS. 2A and 2B may be used to measure the thickness of aconductive layer on a substrate. However, in some implementations, aneddy current sensing system with a higher signal to noise ratio and/orimproved spatial resolution and linearity may be desired. For example,in RTPC applications, obtaining desired cross-wafer uniformity mayrequire an improved eddy current sensing system. FIGS. 3A and 3B show aneddy current sensing system for improved signal to noise ratio andlinearity, while FIGS. 6A, 6B, 7A, and 7B show a core design that may beused for improved spatial resolution. Either or both of these techniquesmay be used to improve eddy current sensing.

FIG. 3A shows an eddy current sensing system 300 that may be morelinear, more stable, and provide a higher signal to noise ratio than thesystems shown in FIGS. 2A and 2B. System 300 includes a coil 320 coupledwith a core 310 (e.g., a generally cylindrically symmetric core, anelongated core such as that shown in FIGS. 5A and 5B or FIGS. 6A and 6B,or other core). In operation, a current generator 330 (e.g., a currentgenerator based on a marginal oscillator circuit) drives the system atthe resonant frequency of an LC tank circuit formed by coil 320 (withinductance L) and a capacitor 315 (with capacitance C). A time-dependentvoltage with amplitude V₀ is rectified using a rectifier 335 andprovided to a feedback circuit 337. Feedback circuit 337 determines adrive current for current generator 330 to keep the amplitude of thevoltage V₀ constant. For such a system, the magnitude of the drivecurrent can be shown to be proportional to the conducting filmthickness. Marginal oscillator circuits and feedback circuits arefurther described in U.S. Pat. No. 4,000,458, which is incorporated byreference, as well as in “Contactless Measurement of SemiconductorConductivity by Radio Frequency-Free-Carrier Power Absorption,” G L.Miller, D. A. H. Robinson and J. D. Wiley, Review of ScientificInstruments, vol. 47, No. 7, July, 1976, which is incorporated byreference.

A number of benefits may be obtained using a system such as system 300.As long as the operating frequency is low enough that the magnetic fieldis not overly attenuated in the conductive region, the drive current islinear with conductive region thickness. Additionally, since theoscillation amplitude is fixed, a highly linear RF rectifier is notnecessary. The signal to noise ratio is improved over other measurementmethods, since the system is operated at the peak of the LC tankresonance curve.

System 300 may also provide fast response (e.g., response times on theorder of about 50 microseconds may be obtained), and may be more simpleto operate and analyze than the implementations of FIGS. 2A and 2B.Finally, system 300 requires a single coil rather than separate driveand sense coils, which reduces complexity and saves winding space.

FIG. 3B shows an implementation of system 300 where current generator330 includes an improved direct current (DC) coupled marginal oscillatorcircuit including a first transistor 340, a second transistor 350, and athird transistor 360. First transistor 340 and second transistor 350form a long-tailed pair: that is, they are substantially identicaltransistors, and a drive current I is alternately switched through firsttransistor 340 and second transistor 350. Third transistor 360 providesDC feedback to the long-tailed pair through a coupling with the base offirst transistor 340. Generally, a large amplitude of oscillation Vo(for example, four volts peak-to-peak) is used. The drive current I isdetermined by measuring the average value of the collector current fortransistor 340.

The marginal oscillator formed using first transistor 340, secondtransistor 350, and third transistor 360 generates a time-dependentcurrent in a coil 370 wound around a core 375. The time-dependentcurrent generates the time-dependent magnetic field that couples with aportion of a conductive layer to provide local thickness information.Feedback is provided using an amplitude stabilization loop 391 includinga rectifier 392, a reference voltage 393, and an integrator 394.Rectifier 392 may be a peak stretcher, and reference voltage 393 may be+2 volts, leading to an RF amplitude of 4 volts peak to peak across theLC tank circuit.

As noted above, when the marginal oscillator operates at the resonantfrequency of the LC tank circuit, the magnitude of the drive currentrequired to maintain a constant V₀ is linearly related to the thicknessof the conductive layer. At resonance, the loss is resistive and can bemodeled as a parallel loss resistance R_(P) 390. R_(P) includes a tankcircuit resistance R_(T) (including, e.g., the resistance of the coilwire), and a sample loading resistance R_(S). The resistances arerelated as shown in Equation (1) below: $\begin{matrix}{\frac{1}{R_{P}} = {\frac{1}{R_{T}} + \frac{1}{R_{S}}}} & {{Equation}\quad(1)}\end{matrix}$

At the resonant frequency of the tank circuit, I, V_(O), and R_(P) arerelated simply by Ohm's law: Vo=IR_(P). Thus, as the sample loadingresistance changes, the drive current necessary to maintain V₀ changes.Thus, the drive current I is a measure of the loading resistance R_(S)and related local thickness of the conductive layer.

The marginal oscillator of FIG. 3B provides a number of advantages overother marginal oscillator designs. First, may be operated at frequenciesranging from DC to the cutoff frequencies of the transistors. Second, itis compatible with high voltage levels (e.g., it may be used withvoltages on the order of volts rather than millivolts). Finally, it isboth simple and stable.

System 300 may be used to monitor a thickness of a conductive layer on awafer according to a process such as a process 400 of FIG. 4. Process400 may be performed, for example, while a wafer with a conductive layeris being polished, and while a core is being translated relative to thewafer. An oscillating magnetic field may be generated using a marginaloscillator circuit, where the oscillating magnetic field is to couplewith a portion of a conductive layer on the wafer (410). For an eddycurrent sensing system positioned proximate to a polishing pad, theoscillating magnetic field extends through the pad and into the portionof the conductive layer.

A voltage V₀ is monitored, where V₀ is the magnitude of a time-dependentpotential difference across a coil and capacitor of an eddy currentsensing system. A drive current for the marginal oscillator is alsomonitored (420). The drive current is adjusted to maintain constant V₀(430). Since the drive current necessary to maintain a constantamplitude is linearly related to the thickness of the conductive layer,the drive current may then be used to determine a polishing endpointand/or a local thickness of the conductive layer (440). Note that theacts in method 400 may be performed continuously and concurrently,although they are listed separately herein.

The eddy current sensing system described above and shown in FIGS. 3Aand 3B may provide enhanced signal to noise ratio, enhanced linearity,and enhanced stability. Additional benefits may be obtained by providingan eddy current sensing system with improved spatial resolution.Improved spatial resolution may be particularly beneficial for RTPC.Obtaining high resolution wafer profile information allows for moreaccurate adjustment of processing parameters, and thus may enablefabrication of devices with smaller CDs. Systems and techniquesdescribed herein provide coil geometries that may be used in a highresolution eddy current system.

One way to increase spatial resolution is to reduce the size of thecore/coil system so that the magnetic field couples to a smaller area ofthe wafer. FIG. 5 show a top view of an eddy current sensing system 500including a core 505 as it sweeps beneath a substrate 510 as a platen530 is rotated. A computer (not shown) may subdivide the sensed eddycurrent signal (raw or processed) into a plurality of sampling zones596. As FIG. 5 illustrates, the spatial resolution of the system islimited by the distance between the protrusions of coil 505 (note thatother parameters such as the distance between the core and theconductive region and the shape of the core may also affect the spatialresolution of the system). Platen 530 may include a flag 540 to besensed by a flag sensor 550 to determine a rotational position of platen530.

Although the spatial resolution may be improved by decreasing the sizeof the core/coil system, it may be difficult to decrease the size of thecore/coil system without suffering unacceptable detriment to themeasurement quality. A number of design considerations may place limitson the minimum core size. For example, desired values of frequency,dynamic impedance, and quality factor may place limits on the minimumcore size.

The range of desired frequencies for eddy current sensing may be chosenbased on a response time considerations (higher frequencies enablefaster response), and on skin depth considerations. As the frequency ofthe electromagnetic radiation (i.e., the frequency of the magneticfield) increases, the skin depth (a measure of the distance that themagnetic field penetrates) decreases. In order to accurately measure thethickness of a layer, the magnetic field should penetrate the entirethickness.

Limitations on both the quality factor and the dynamic impedance preventthe need to switch inconveniently large currents in the electronic loop.

Table 1 shows some desired values for frequency, dynamic impedance, andquality factor that may limit the minimum core size. The frequency valuein Table 1 is based on a copper film up to about 1.5 microns thick; forother materials and/or thicknesses, different frequency values may beappropriate. Note that L represents the inductance of a coil such ascoil 370 of FIG. 3B, C represents a capacitance of a capacitor such ascapacitor 380 of FIG. 3B, Z represents the unloaded dynamic impedance ofa coil and capacitor system, and R_(P) represents a parallel lossresistance, which includes both the parallel loss resistance of the LCcircuit and of the conductive layer. TABLE 1 Design Parameter ValueGuideline Frequency $\frac{1}{2\pi\sqrt{LC}}$ ≦350 kHz Dynamic Impedance$\sqrt{\frac{L}{C}}$ ≧100 ohms Quality Factor $\frac{R_{P}}{Z}$ ≧10

Design guidelines such as those listed in Table 1 may be difficult (orimpossible) to achieve for small cores. For example, as the size of thecore is decreased, finer wires with increased series loss resistance aregenerally required. Thus, the Q-factor of the system decreases as thesize of the core is decreased.

The current inventors recognized that one can trade off spatialresolution in perpendicular directions by using a core that is long inone direction and narrow in another. FIGS. 6A and 6B show side and topviews of a coil/core system 600 that may be used to provide highresolution eddy current measurements without a significant detriment tothe Q-factor of the system. An elongated core 610 is generally “E”shaped; that is, it has three protrusions extending upward from a backportion. As shown in FIG. 6B, core 610 extends a length L that isgreater than the width W of core 610. A coil 620 may be wound around thecenter protrusion. Coil 620 may be coupled with a capacitor 630. Inimplementations of eddy current sensing systems such as system 200 ofFIGS. 2A and 2B, separate sense and drive coils may be used.

In some implementations, a coil such as coil 620 may be litz wire (wovenwire constructed of individual film insulated wires bunched or braidedtogether in a uniform pattern of twists and length of lay), which may beless lossy than solid wire for the frequencies commonly used in eddycurrent sensing. Core 610 may be a MnZn ferrite, or may be a NiZnferrite. Core 610 may be coated. For example, core 610 may be coatedwith a material such as parylene to prevent water from entering pores incore 610, and to prevent coil shorting.

FIGS. 7A and 7B show side and top views of a different coil/core system700 that may alternately be used to provide high resolution eddy currentmeasurements without a significant detriment to the sensed signal. Anelongated core 710 can be “U” shaped; that is, it has two protrusionsextending upwards from the ends of a back portion. As shown in FIG. 7B,core 710 extends a length L that is larger than the width W of core 710.A coil 720 may be wound around the two protrusions and may be coupledwith a capacitor 730. Again, for implementations such as those shown inFIGS. 2A and 2B, separate drive and sense coils may be used. FIG. 7Cshows a side view of an alternate winding scheme for a U-shaped core. Asshown in FIG. 7C, coil 720 may be wound between the two protrusions in a“FIG. 8” configuration.

In some implementations, the core may be shielded to more preciselydirect the flux lines toward a particular portion of a conductive layerand thus to improve spatial resolution. Note that shielding the core mayresult in a reduction of the Q-factor, and thus the shieldingconfiguration shown should provide sufficient direction of flux lineswithout too much detriment to the Q-factor. FIGS. 8A through 8C showdifferent shielding configurations that may be used. FIG. 8A shows aside view of a shield 810 proximate to a core 800. Shield 810 may have agap (not shown) so that eddy currents are not induced in shield 810 dueto the time-dependent magnetic field generated by the eddy currentsensing system. Shield 810 may be made of sheet aluminum. FIG. 8B showstop and side views of a shield 820, which may also be made of sheetaluminum. The top view shows a gap 825 to prevent eddy currentgeneration in shield 820. FIG. 8C shows a top view of a shield 830formed using copper tape. A gap 835 between a first end 836 and a secondend 837 of the copper tape prevents generation of eddy currents inshield 830.

FIGS. 9A and 9B show top and side views of the relative position of asubstrate 920 with respect to an elongated core 910 (which may besimilar to core 610 of FIGS. 6A and 6B or core 710 of FIGS. 7A and 7B).For a scan through a slice A-A′ through the center of a wafer 920 havinga radius R, core 910 is oriented so that its long axis is perpendicularto a radius of wafer 920. Core 910 is translated relative to thediameter of the wafer as shown. Note that the magnetic field produced bya coil wound around core 910 induces eddy currents in a conductiveregion that is elongated in shape as well, with a length greater than awidth. However, the length and the width are generally not the same asthe length and width of core 910, and the aspect ratio and cross sectionof the conductive region is generally different than that of core 910 aswell.

Although the configuration of FIGS. 9A and 9B may provide improvedresolution for most of slide A-A′ of wafer 920, as core 910 translatesalong the first and last segments 930 of the radius, a portion of core910 is not proximate to the substrate. Therefore the measurement forsegments 930 is less accurate and may place a limit on the maximumdesirable length L of core 910. Additionally, as core 910 approaches thecenter of wafer 920, it is sampling a larger radial range. Therefore,the spatial resolution for a particular radial distance r=R issignificantly better than the spatial resolution of r≈0.

As explained above, the length L of core 910 is greater than its widthW. That is, the aspect ration L/W is greater than one. Different valuesfor L, W, and L/W may be used for different implementations. Forexample, W may range from a fraction of a millimeter to more than acentimeter, while L may range from about a millimeter (for smallervalues of W) to ten centimeters or greater.

In a particular implementation, W is between about a millimeter andabout ten millimeters, while L is between about one centimeter to aboutfive centimeters. More particularly, a coil such as coil 610 of FIGS. 6Aand 6B may be about five millimeters wide, with each protrusion beingabout a millimeter in width and with each space between adjacentprotrusions being about a millimeter. The length may be about twentymillimeters. The height may be about five millimeters and may beincreased if desired to allow for more coil turns. For a coil such ascoil 710 of FIGS. 7A and 7B, the length may be about two centimeters andthe width may be about 2.5 millimeters. Each protrusion may be about onemillimeter in width, and the space between the protrusions may be about1.5 millimeters. The height may be about three millimeters. Of course,the values given here are exemplary; many other configurations arepossible.

In some implementations, the long axis of an elongated core may not beexactly perpendicular to a radius of a substrate. However, an elongatedcore may still provide improved resolution over available coregeometries, particularly near the wafer edge. FIG. 10A shows animplementation in which an elongated core 1010 is positioned underneatha platen 1020. Prior to sweeping underneath a substrate 1030, core 1010is at position 1015. At position 1015, core 1010 is positionedapproximately perpendicular to a radius of substrate 1030. Therefore,for r≈R, the portion of a conductive layer that couples with themagnetic field produced by the coil wound around core 1010 is generallyat the same radial distance from the center of the wafer. Note that bothplaten 1020 and substrate 1030 are both rotating as core 1010 sweepsbeneath substrate 1030, and that the wafer may also sweep with respectto platen 1020, as indicated. Additionally, a flag 1040 and a flagsensor 1050 may be used to sense the rotational position of platen 1020.

FIG. 10B shows a close up of wafer 1030 as core 1010 sweeps below wafer1030. At a first position 1012, core 1010 measures the thickness at aradius r≈R. However, at a position 1014, core spans a range of radiifrom r₁ to r₂. Therefore, the spatial resolution at the outer edge ofwafer 1030 is much better than the spatial resolution near the center ofwafer 1030. Note that this effect is reduced as the length L of core1010 is decreased.

As noted above, spatial resolution also depends on the distance betweenthe core and the conductive layer. FIG. 11 shows a side view of a system1100 providing close proximity between a core, as well as preventingfluid from leaking. A core 1110 is coupled with a coil 1120 forproducing a time-dependent magnetic field to induce eddy currents in aconductive region on a wafer (not shown). Core 1110 and coil 1120 arefixed within a sensor housing 1130. Sensor housing 1130 both protectscore 1110 and coil 1120 from fluid and positions it with respect to thewafer. Housing 1130 is coupled with an upper platen 1150 via an o-ringseal 1140 to prevent leaking. A pad assembly 1155 includes a sub-pad1160, a pad 1170, and a pad window 1180, which includes a thinnedportion 1185. Thinned portion 1185 allows core 1110 to be positioned inclose proximity to the wafer. For example, the distance between the topof core 1110 may be about 50 mils. Note that other configurations may beused; particularly, pad configurations without a sub-pad and/or withouta pad window may be used.

FIG. 12 shows a chemical mechanical polishing apparatus 20 that may beused with an eddy current sensing system such as those described above.A description of a similar polishing apparatus 20 can be found in U.S.patent application Ser. No. 09/900,664, the entire disclosure of whichis incorporated herein by reference. FIG. 13 shows how a core 42 (e.g.,a core such as core 610 of FIGS. 6A and 6B, core 710 of FIGS. 7A and 7B,or other core) may be positioned with respect to a polishing pad 30having a thinned section 36 (and thus positioned with respect topolishing station 22 of apparatus 20).

Referring to FIGS. 12 and 13, one or more substrates 10 can be polishedby CMP apparatus 20. Polishing apparatus 20 includes a series ofpolishing stations 22 and a transfer station 23. Transfer station 23transfers the substrates between the carrier heads and a loadingapparatus.

Each polishing station includes a rotatable platen 24 on which is placeda polishing pad 30. The first and second stations can include atwo-layer polishing pad with a hard durable outer surface or afixed-abrasive pad with embedded abrasive particles. The final polishingstation can include a relatively soft pad. Each polishing station canalso include a pad conditioner apparatus 28 to maintain the condition ofthe polishing pad so that it will effectively polish substrates.

A rotatable multi-head carousel 60 supports four carrier heads 70. Thecarousel is rotated by a central post 62 about a carousel axis 64 by acarousel motor assembly (not shown) to orbit the carrier head systemsand the substrates attached thereto between polishing stations 22 andtransfer station 23. Three of the carrier head systems receive and holdsubstrates, and polish them by pressing them against the polishing pads.Meanwhile, one of the carrier head systems receives a substrate from anddelivers a substrate to transfer station 23.

Each carrier head 70 is connected by a carrier drive shaft 74 to acarrier head rotation motor 76 (shown by the removal of one quarter ofcover 68) so that each carrier head can independently rotate about itown axis. In addition, each carrier head 70 independently laterallyoscillates in a radial slot 72 formed in carousel support plate 66. Adescription of a suitable carrier head 70 can be found in U.S. Pat. No.6,422,927, the entire disclosure of which is incorporated by reference.In operation, the platen is rotated about its central axis 25, and thecarrier head is rotated about its central axis 71 and translatedlaterally across the surface of the polishing pad.

A slurry 38 containing a reactive agent (e.g., deionized water for oxidepolishing) and a chemically-reactive catalyzer (e.g., potassiumhydroxide for oxide polishing) can be supplied to the surface ofpolishing pad 30 by a slurry supply port or combined slurry/rinse arm39. If polishing pad 30 is a standard pad, slurry 38 can also includeabrasive particles (e.g., silicon dioxide for oxide polishing). A recess26 is formed in platen 24, and a thin section 36 can be formed inpolishing pad 30 overlying recess 26. Aperture 26 and thin pad section36, if needed, are positioned such that they pass beneath substrate 10during a portion of the platen's rotation, regardless of thetranslational position of the carrier head.

As shown in FIG. 13, CMP apparatus 20 can also include a position sensor80, such as an optical interrupter, to sense when core 42 is beneathsubstrate 10. For example, the optical interrupter could be mounted at afixed point opposite carrier head 70. A flag 82 may be attached to theperiphery of the platen. The point of attachment and length of flag 82is selected so that it interrupts the optical signal of sensor 80 whilecore 42 sweeps beneath substrate 10. Alternately, the CMP apparatus caninclude an encoder to determine the angular position of the platen.

Referring to FIG. 13, an eddy current monitoring system 40 may includedrive and feedback circuitry 50, including an oscillator such as amarginal oscillator described above and shown in FIGS. 3A and 3B. Thecore 42 and the coil 44 of the eddy current sensing system located belowthin section 36 of polishing pad 32 sweep beneath the substrate witheach rotation of the platen. Note that although a single coil 44 isshown here, in some implementations separate drive and sense coils areused, and separate sensing circuitry is provided. Circuitry 50 maybelocated apart from platen 24, and can be coupled to the components inthe platen through a rotary electrical union 29.

A computer 90 can receive measurements from circuitry 50, and can beprogrammed to divide the measurements from each sweep of the corebeneath the substrate into a plurality of sampling zones (e.g., samplingzones 596 of FIG. 5), to calculate the radial position of each samplingzone, to sort the measurements into radial ranges, to determine minimum,maximum and average measurements for each sampling zone, and to usemultiple radial ranges to determine the polishing endpoint, as discussedin U.S. Pat. No. 6,399,501, filed Dec. 13, 1999, issued Jun. 4, 2002,the entirety of which is incorporated herein by reference. Note that themeasurements may be amplitude measurements, phase measurements, and/ordrive current measurements, depending on the configuration of system 40.Output from computer may be displayed on an output device 92 duringpolishing to permit a user to visually monitor the progress of thepolishing operation.

Moreover, after sorting the eddy current measurements into radialranges, information on the metal film thickness can be fed in real-timeinto a closed-loop controller to periodically or continuously modify thepolishing pressure profile applied by a carrier head, as discussed inU.S. Patent Application Ser. No. 60/143,219, filed Jul. 7, 1999, theentirety of which is incorporated herein by reference. For example, thecomputer could determine that the endpoint criteria have been satisfiedfor the outer radial ranges but not for the inner radial ranges. Thiswould indicate that the underlying layer has been exposed in an annularouter area but not in an inner area of the substrate. In this case, thecomputer could reduce the diameter of the area in which pressure isapplied so that pressure is applied only to the inner area of thesubstrate, thereby reducing dishing and erosion on the outer area of thesubstrate. Alternatively, the computer can halt polishing of thesubstrate on the first indication that the underlying layer has beenexposed anywhere on the substrate, i.e., at first clearing of the metallayer.

A number of implementations have been described. Nevertheless, it willbe understood that various modifications may be made without departingfrom the spirit and scope of the invention. For example, different coilgeometries may be used. The core may be positioned differently withrespect to the platen and substrate than described. Although elongatedcores with generally rectangular cross section are shown, otherconfigurations may be used. For example, ovoid cross sections may beused, where the “length” then refers to the long axis and the “width”refers to the short axis. The acts in the processes shown in FIGS. 1 and4 need not necessarily be performed in the order shown. Accordingly,other embodiments are within the scope of the following claims.

1. An apparatus for chemical mechanical polishing, comprising: a platento support a polishing surface; and an eddy current monitoring system togenerate an eddy current signal, the eddy current monitoring systemcomprising: an elongated core positioned at least partially in theplaten, the elongated core having a length and a width, the lengthlonger than the width.
 2. The apparatus of claim 1, wherein theelongated core comprises a back portion and one or more protrusionsextending perpendicularly from the back portion towards the polishingsurface.
 3. The apparatus of claim 2, further including a coil coupledwith at least one of the one or more protrusions.
 4. The apparatus ofclaim 3, wherein the coil comprises woven wire.
 5. The apparatus ofclaim 3, wherein the one or more protrusions include a first protrusionand a second protrusion, and wherein the coil is coupled with the firstprotrusion and the second protrusion in a figure eight configuration. 6.The apparatus of claim 3, further including another coil coupled withthe back portion.
 7. The apparatus of claim 1, wherein the length is atleast twice the width.
 8. The apparatus of claim 1, wherein the lengthis between about five millimeters and about ten centimeters.
 9. Theapparatus of claim 1, wherein the width is less then about a centimeter.10. The apparatus of claim 1, wherein the eddy current monitoring systemfurther includes a shield positioned proximate an outer surface of theelongated core.
 11. The apparatus of claim 10, wherein the shieldincludes a gap.
 12. An eddy current sensing system, comprising: anelongated core having a length and a width, the length longer than thewidth; a housing having mounting features shaped and configured toposition the elongated core in a recess of a platen; a coil wound arounda portion of the elongated core; a drive system to generate a current inthe coil; and a sense system to derive a characteristic of a conductiveregion based on eddy currents generated in the conductive region. 13.The system of claim 12, wherein the elongated core comprises a backportion and one or more protrusions extending perpendicularly from theback portion towards the polishing surface
 14. The system of claim 13,further comprising a coil coupled with at least one of the one or moreprotrusions.
 15. The system of claim 12, wherein the elongated corecomprises a ferrite material.
 16. The system of claim 15, wherein theferrite material is chosen from the group consisting of a MnZn ferritematerial and a NiZn ferrite material.
 17. The system of claim 12,wherein the elongated core is coated with a material.
 18. The system ofclaim 17, wherein the material comprises parylene.
 19. The system ofclaim 17, further comprising: the platen, including complementarymounting features to receive the housing; a polishing pad having apolishing surface mounted to the platen, such that when the mountingfeatures of the housing engage with complementary features of the patenta top surface of one of the one or more protrusions of the elongatedcore is positioned about two millimeters or less from the polishingsurface of the pad.
 20. The system of claim 19, wherein the top surfaceis positioned between about one millimeter and about two millimetersfrom the polishing surface.
 21. The system of claim 19, wherein theelongated core has a generally U-shaped cross section.
 22. The system ofclaim 19, wherein the elongated core has a generally E-shaped crosssection.
 23. A method of in-situ profile control comprising: processinga conductive layer on a wafer using a plurality of processingparameters; inducing eddy currents in a first region of a conductivelayer on a wafer, the first region having a length greater than a width;and acquiring measured thickness data for the conductive layer in thefirst region, the thickness data based on the eddy currents induced inthe first region.
 24. The method of claim 23, further including:inducing eddy currents in a second region of the conductive layer, thesecond region having the length and the width; and acquiring measuredthickness data for the conductive layer in the second region, thethickness data based on the eddy currents induced in the second region.25. The method of claim 24, further comprising comparing the measuredthickness data for the conductive layer in the first region and thesecond region to a desired thickness profile to determine a profileerror.
 26. The method of claim 25, further comprising changing at leastone of the processing parameters based on the profile error.
 27. Themethod of claim 23, wherein the length is at least twice the width. 28.The method of claim 23, wherein the width is about a millimeter or less.29. The method of claim 23, wherein the width is between about onemillimeter about three millimeters.
 30. The method of claim 23, whereinthe eddy currents are generated in response to a time-dependent magneticfield generated with a coil coupled with an elongated core.
 31. Themethod of claim 23, wherein acquiring measured thickness data for theconductive layer in the first region comprises acquiring amplitude databased on an amplitude of a sense signal in a sense coil.
 32. The methodof claim 23, wherein acquiring measured thickness data for theconductive layer in the first region comprises acquiring phase databased on a phase of a sense signal in a sense coil.
 33. The method ofclaim 23, wherein acquiring measured thickness data for the conductivelayer in the first region comprises acquiring drive current data basedon a drive current to maintain a constant voltage across a coil and acapacitor, the coil and the capacitor included in a circuit to generatea time-dependent magnetic field to induce the eddy currents in the firstregion.
 34. A semiconductor processing apparatus, comprising: a directcurrent (DC) coupled marginal oscillator to generate a time-dependentdrive current; a coil to generate a time-dependent magnetic field tocouple with a portion of a conductive region on a wafer, the marginaloscillator comprising: a first transistor and a second transistorcomprising a long-tailed pair; a third transistor, the third transistorcoupled with the first transistor to provide DC feedback through a baseof the first transistor.
 35. The apparatus of claim 34, wherein themarginal oscillator is to generate the time-dependent drive current at aresonant frequency of a circuit, the circuit comprising the coil coupledwith a core and a capacitor.
 36. The apparatus of claim 35, wherein thecore is an elongated core.
 37. The apparatus of claim 35, wherein thecore is generally cylindrically symmetric.
 38. The apparatus of claim34, wherein the third transistor provides direct current feedback to thebase of the first transistor to cause the marginal oscillator togenerate the time-dependent drive current such that a potentialdifference across the coil and the capacitor is maintained at agenerally constant amplitude.
 39. The apparatus of claim 38, furtherincluding a feedback circuit to sense the amplitude of the potentialdifference across the coil and the capacitor.
 40. A method comprising:generating a time-dependent current at a resonant frequency of a circuitcomprising a coil coupled with a core and a capacitor, thetime-dependent current generated by a marginal oscillator having a firsttransistor and a second transistor comprising a long-tailed pair;inducing eddy currents in a first region of a conductive layer on awafer, wherein the eddy currents are induced by a time-dependentmagnetic field produced by the coil; determining an amplitude of apotential difference across the coil and the capacitor; adjusting thetime dependent drive current based on direct current feedback from athird transistor coupled to the base of the first transistor to maintaina desired amplitude of the potential difference; and determining one ormore parameters of the first region based on the drive current.
 41. Themethod of claim 40, wherein the one or more parameters of the conductiveregion include a thickness of the conductive region.
 42. The method ofclaim 40, wherein the one or more parameters of the conductive regioninclude an endpoint of a process for polishing the conductive region.43. The method of claim 40, further comprising translating the core withrespect to the wafer so that the time-dependent magnetic field induceseddy currents in a second region of the conductive layer.
 44. The methodof claim 43, further comprising: determining a subsequent amplitude ofthe potential difference across the coil and the capacitor; adjustingthe time dependent current of the first transistor and the secondtransistor based on direct current feedback from a third transistorcoupled to the base of the first transistor to maintain the desiredamplitude of the potential difference; and determining one or moreparameters of the second region based on the drive current.
 45. Themethod of claim 44, wherein the one or more parameters of the firstregion include a thickness of the first region, and wherein the one ormore parameters of the second region include a thickness of the secondregion, and further comprising comparing the thickness of the firstregion and the thickness of the second region to a desired thicknessprofile to determine a profile error.
 46. The method of claim 45,further comprising adjusting one or more processing parameters based onthe profile error.
 47. An apparatus for semiconductor processing,comprising: a wafer carrier; an elongated core positioned proximate tothe wafer carrier, the elongated core having a length and a width, thelength longer than the width; a coil wound around a portion of theelongated core; a drive system to generate a current in the coil, thecurrent to produce a time-varying magnetic field; and a sensing systemto derive a characteristic of a wafer positioned in the wafer carrierbased on eddy currents generated in a conductive portion of the wafer,the eddy currents generated in response to the time-varying magneticfield.
 48. The apparatus of claim 47, wherein the elongated corecomprises a back portion and one or more protrusions extendingperpendicularly from the back portion towards the wafer carrier.
 49. Theapparatus of claim 48, wherein the one or more protrusions include afirst protrusion and a second protrusion, and wherein the coil iscoupled with the first protrusion and the second protrusion in a figureeight configuration.
 50. The apparatus of claim 47, further including atranslation mechanism to translate the elongated core with respect tothe wafer carrier.
 51. The apparatus of claim 47, wherein the wafercarrier comprises a polishing pad mounted to a platen.
 52. The apparatusof claim 47, wherein the drive system comprises a marginal oscillator.53. The apparatus of claim 52, wherein the sensing system comprises afeedback circuit to determine a drive current of the marginaloscillator.